A percussion primer is a critical energy transducer used for converting a mechanical impetus into rapid chemical energy for most armament ignition trains, in particular for small arms ammunition used by both commercial and military sectors. Primers contain impact and friction sensitive mixes adapted to the armament system of design and are usually composed of common explosives and pyrotechnic components that have gradually evolved over time.
Over 100 years ago, mercury fulminate-based formulations were the most commonly used primer mixes. In the 1920s, alternate priming mixes were found to replace mercury fulminate, as this previous composition was found to deteriorate rapidly under tropical conditions and cause potential health problems or concerns such as lethargy and nausea to the shooter after firing. However, the alternate mixes that were based on lead thiocyanate/potassium chlorate formulations, were soon recognized as detrimental to weapon barrels because of the formation of corrosive water soluble potassium chloride salts upon combustion. Later primer mixes developed in the 1950s were based on the primary explosive lead styphnate (in its normal form: lead(II) 2,4,6-trinitro-m-resorcinate), a substance, which is much, more stable than mercury fulminate.
Currently, the most commonly used primer mixes are chemical mixtures comprising at least a primary explosive, and pyrotechnic mixes comprising oxidizing agents and fuel source.
Primary explosives are essential components in primers as they are directly responsible for converting a relatively small stimulus (i.e., a small quantity of energy in the form of an impact, friction, shock, heat, or electrostatic discharge) into a large chemical energy release capable of igniting the fuel/oxidizer components of the mixture. In the case of primer formulations, the most common primary is lead styphnate (also known as lead trinitrorescorcinate), which has two commercially used chemical forms: “normal” lead styphnate (lead(II) 2,4,6-trinitro-m-phenylene dioxide, C6HN3O8Pb), and “basic” lead styphnate (lead(II) hydroxide 2,4,6-m-phenylene dioxide, C6H3N3O10Pb2). Both forms possess very similar performance and are often used interchangeably. Either form of lead styphnate generally requires the addition of a second primary explosive, tetrazene, in the formulation to enhance the primer's reliability.
Lead styphnate a primary explosive, may be combined with the weaker but more sensitive explosive tetrazene (1-(5-tetrazolyl)-3-guanyl tetrazene hydrate, also sometimes referred to as tetracene) to render the lead styphnate based primer composition sufficiently sensitive to percussion.
Unfortunately, regardless of such desirable explosive properties, the lead found in lead styphnate is both an acute and chronic toxin, and the human body has difficulty in removing it once it has been absorbed and dissolved in the blood. Also, the manufacturing of these lead-based primary explosives results in the production of significant quantities of highly toxic hazardous waste. Handling and use of these lead base primary explosives results in further exposure, as well as, the wide dissemination of lead containing combustion by-products.
Other additives in primer compositions also contribute to toxic effects in humans. For instance, barium nitrate, a common oxidizing agent chosen primarily for its low hygroscopicity can cause cardiac arrest and respiratory failure leading to death.
Numerous attempts have been made towards producing an effective, non-toxic primer composition. As used herein “non-toxic” is intended to mean a substance consisting essentially of materials that are not toxic heavy metals (such as lead or barium, known carcinogens or poisons, especially when vaporized, burnt or exploded as in the firing of an ammunition round).
In the production of non-toxic primer mixes, 2-diazo-4,6-dinitro phenol (DDNP) is often a preferred lead-free substitute for lead styphnate as the primary explosive. Like lead styphnate, DDNP typically is accompanied by tetrazene as a secondary primary explosive to render the composition sufficiently sensitive to percussion. However, DDNP-based mixes have not found military use in the U.S. and have largely been relegated to practice ammunition in the commercial sector. The primary reason for the lack of utility is DDNP's poor performance at low temperatures.
Alternate primer mix designs have focused on eliminating the primary explosive as well as ability to be manufactured through automation such as red phosphorus (RP) based primers and the nanothermite-based Metastable Intermolecular Composites (MIC) developed in the last 12 years for military primers. Red phosphorous reacts slowly with air and moisture to form highly toxic phosphine gas, and further can corrode brass, which is the most commonly used material for ammunition casings. MICs require metal nano-powders to function, and therefore possess significant inhalation safety concerns; they are further almost always based on nano-aluminum, which degrades in the presence of moisture and can have pyrophoric qualities.
U.S. Pat. No. 7,833,330, by Fronabager et al., discloses the use of DBX-1 (copper(I) 5-nitrotetrazolate) as a potentially useful-lead free substitute for lead azide. It is not hygroscopic, does not contain highly toxic metals, is thermally stable, functions well at ambient and cold temperatures, and possesses comparable sensitivity to lead azide and lead styphnate. Given that conventional wisdom in the field has held that lead azide, being a more brisant, less-heat-generating compound compared to lead styphnate, was not suitable for use in primer applications where flame, high temperature, and slag are desired outputs.
The United States Military armaments are subjected to rigor of high standard performance, stability, and cost assessments. Commercial primer manufacturers are attempting to develop non-toxic primer compositions to meet the strict evaluations set by the military. With a recent development of a direct lead azide replacement, DBX-1, and concurrent pyrotechnic exploration with new materials of commercial and military interest, primer mix design expands into new territory.
So, while there has been some progress in establishing a lead-free replacement for lead azide, there is still a need in the art for a lead-free, i.e., environmentally green (safe) replacement for the primer composition as a whole.